In positive displacement compressors, discrete volumes of gas are trapped and compressed with the trapped, compressed volumes being discharged from the compressor. The trapping of the volumes at suction pressure and their discharge at discharge pressure each produce pressure pulsations and the related noise generation. While mufflers can be made to attenuate noise in a particular frequency range, or ranges, variable speed compressors may operate over ranges beyond the effective range(s) of conventional absorptive mufflers. This may be due to operating at rotational speeds outside the peak performance region of the absorptive device or at speeds where absorptive techniques are inadequate e.g. at frequencies well below the quarter wave thickness of the absorptive material. Accordingly, there would be no effective attenuation of a variable speed positive displacement compressors over some ranges of normal operation where conventional absorptive mufflers are employed.
The flow of gas through a muffler is along a flow path defined by the pressure differential across the muffler. The direction of noise generation is not dictated by the flow direction. Reflected sound energy is generated each time there is a change in the cross section of the flow path with some of the sound energy being reflected in the opposite direction to that of the gas flow. It is through this mechanism that xe2x80x9creactivexe2x80x9d type mufflers are designed to attenuate specific frequencies. In an absorptive muffler a portion of the flow path is defined by an absorptive material overlain by perforate metal, or the like. There is a trade off between flow resistance and noise reduction, with respect to the length and cross section of the flow path, in designing the muffler. Typical performance is limited by the relationship of the flow passage length to its height/minimum spacing in an absorptive device with peak attenuation occurring at a frequency related to the depth and impedance characteristics of the liner material.
The present invention is directed to an absorptive/reactive muffler including a central cylindrical section having an opening, preferably, at the downstream end and containing a plurality of Helmholtz resonators, a mix of quarter and half wave resonators with each of the resonators being turned to a slightly different frequency to provide wider bandwidth attenuation characteristics or a combination of Helmholtz and quarter and/or half wave resonators. The central cylindrical section is serially overlain by an absorptive material and a first perforate material. The perforate material defines the inner surface of the flow path. A second perforate annular surface is underlain with an absorptive material and is spaced from the first perforate material and coacts therewith to define the fluid flow path. Noise traveling along the fluid flow path reflects between the two surfaces of absorptive material overlain by the perforate material and is attenuated by the absorptive material. Upon reaching the end of the annular flow path, the impedance discontinuity defined by the change in flow cross section directs some of the generated noise into the central cylindrical section containing the resonators. If necessary, or desired, the outer annular surface partially defining the annular flow path may be smooth rather than lined with absorptive material overlain by perforate material.
It is an object of this invention to provide performance enhancement over conventional absorptive mufflers.
It is a further object of this invention to provide a muffler having enhanced performance in a plurality of narrow frequency bands. These objects, and others as will become apparent hereinafter, are accomplished by the present invention.
Basically, the preferred muffler includes an annular flow path for the gas with the center of the annulus having a plurality of resonators which are in open communication with the downstream end of the annular flow path. The flow path is at least partially lined by an absorptive material overlain by a perforate material.